In recent years, an explosion of immunological data has been amassed that reveals an unexpected variety of roles played by the many gene products of the MHC, particularly in defense responses to attack of a host of microbial and viral pathogens. Our goal is to understand at the molecular level how the host can effectively respond to such microbial attacks and, when these systems are compromised, what is the structural basis of transplant rejection, autoimmune disease and tumor development. NK and T cells are key components of the cellular immune system. This proposal encompasses a number of established biochemical, immunological, and biophysical approaches to improve our understanding at the structural level of the range of classical and non-classical MHC and MHC ligands that can be recognized by these T and NK cell receptors. Such structural information can be used to interpret the vast database of immunological data and aid in developing novel peptides, glycopeptides, and glycolipid-based therapeutics to combat microbial diseases. Specifically, we propose to determine the three-dimensional structures of: 1) Non-classical CD1d and CD1b unbound or in complex with self and non-self lipid/glycolipid antigens to understand how these MHC-like receptors combat microbial infections. 2) Murine MHC class I H-2Kb in complex with a number of mucin-derived peptides, as part of an international collaboration to develop peptide-based vaccines for cancer immunotherapies. 3) Murine MHC class II I-Ag7 to probe the unusual physicochemical properties of the P9 peptide binding pocket, unique amongst I-A alleles, and address what self-antigen peptides correlate with autoimmune diabetes. 4) Murine NK cell NKG2D in complex with its high affinity ligand H60, that is up-regulated on various tumor cells, and hence, a potential target for design of cancer immunotherapeutics. 5) Chaperones from the rat system, including calnexin, calreticulin, tapasin, Erp57 and TAP transporter to probe the pathway responsible for class I peptide loading in the ER and processing of non-classical MHC class I molecules. 6) Murine Qa-1b and rodent RT-BM1, both complexed with the protective Qdm signal peptide and nonprotective hsp60-derived signal peptides, produced and bound during cellular stress events.